I am working on a hat and the pattern says to "work 3 rows even in stocking st." I know that a stocking st. you knit a row then purl a row, so on and so forth. However, being an odd number of rows even in stocking st. do I knit a row, purl a row, then knit a row? Please help clarify my pattern! Thanks!

Assuming you're working flat and not in the round, yes, alternate knit rows with purl rows, even or odd number of rows doesn't matter. If your first row is a knit row, it will be 1 row knit, 1 row purl, 1 row knit; if the first is a purl row, it will be 1 row purl, 1 row knit, 1 row purl. The pattern writer will have taken this into consideration and the next row will be written appropriately.

If you're working in the round, all rows are knit for stockinette. I mention this because many hat patterns are worked in the round.

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Yes, this is likely deliberately done to put you onto a particular side of the hat, right side or wrong side. What are the rows before this direction? Can you link to a pattern?

Purl, knit, purl the 3 stockinette rows.
Ok so the instructions give you a decrease round (K2. K2tog. *K15. Sl1. K1. psso. K1. K2tog. Rep from *can 3 times more. K15. Sl1. K1. psso. K1. 92 sts.
Then you would work "Work 3 rows even in stocking st. " That's purl a row, knit a row and then purl a row. You're now set to knit on the NEXT ROW: K2. K2tog. *K13. Sl1. K1. psso. K1. K2tog. Rep from * 3 times more. K13. Sl1. K1. psso. K1. 82 sts.
Then "Work 3 rows even in stocking st. " So that's purl a row, knit a row, purl a row.
Each time the decreases are on a knit row and you're working purl, knit and purl rows so that you're ready for a decreases row on another knit row. That'll keep the knit side of the stockinette on the outside which is usually the pattern.

The decs are done on a knit row, so your next row would be purl; then you knit one row, purl one row, and you're back to the next dec row which is also knit. Just think of it as doing the decrease row every 4th row with 3 plain rows between.

Ok. Thanks everyone! I have those rows completed. Now i have just finished a row of purling. The next instructions say to "cont in this manner, dec 10 sts every altmonth row, until 22 sts rem." My question is do I stocking st? and when and what decreasing sts do I use?

Yes continue in stockinette st and if possible, continue the decrease lines that are forming up the crown of the hat. There's a decrease of 2sts in the bolded knit sts with each dec row. If you can, follow that pattern, decreasing to K11 and then K9 and so on
*K15. Sl1. K1. psso. K1. K2tog. Rep from *can 3 times more. K15. Sl1. K1. psso. K1. 92 sts. "Work 3 rows even in stocking st. " NEXT ROW: K2. K2tog. *K13. Sl1. K1. psso. K1. K2tog. Rep from * 3 times more. K13. Sl1. K1. psso. K1. 82 sts.